1. Field of the Invention
The field of invention relates to the stimulation of oil and gas wells. More specifically, the field relates to the use of encapsulated acids for stimulating oil and gas wells.
2. Description of the Related Art
For carbonate hydrocarbon-bearing formations having fractures, fissures and other natural and man-made hydrocarbon conduits through the formation, stimulation by conventional fracture acid treatments (strong mineral acids like HCl, H2SO4, HF, and HNO3) have experienced challenges. The significant problem with directing conventional fracture acid treatments though such fractures or fissures to improve production and hydrocarbon fluid flow is due to the reactivity of the acid itself with carbonates. Unless the carbonate has been passivized or coated with a neutral material, the acid or solution of acid reacts with the portion of the hydrocarbon-bearing formation that it immediately contacts: the portion proximate to the wellbore. Reacting close to the face or wall of the wellbore achieves little to no fracture conductivity improvement along the entire fracture length or through the hydrocarbon-bearing formation.
Traditional methods to retard or delay the reaction of strong mineral acids include acid gelation, acid-in-oil (W/O) emulsification and adsorption of surfactants on the rock face. Each of these techniques has limitations. The acid-in-oil emulsification and the acid gelation exist in a metastable/unstable state that may easily break down without any controlled mechanism of release. Other limitations include reduction in acid efficiency; increase in cost and complexity due to the additives, especially surfactants; poor control over penetration depth thought the formation; and the need to use corrosion inhibitors as some of the additives attach the metals of the casing and well tools.
Acid fracturing with an acid-in-oil (W/O) emulsification is a technique where the strong mineral acid is surrounded by a hydrocarbon liquid such as diesel. The diesel provides a liquid hydrophobic barrier that upon contact with a sharp or hydrocarbon-bearing surface, or upon introduction of an emulsion breaker, would permit the acid solution to be released. Once the emulsion breaks the strong mineral acid is able to react with the carbonate rock. The two main problems with this technique are that there is a lack of control over the acid release and the questionable shear resistance of the emulsion itself. The result of such an application is a less localized, moderately deeper into the formation but still unevenly distributed fracture conductivity improvement.
Other alternatives include the use of weaker organic acids, including citric acid. The results have been mixed. In general, weaker organic acids exhibit a significantly lower bulk dissolution capacity than strong mineral acids. They are generally more expensive per equivalent acid volume. Organic acids, however, do demonstrate an ability to perform such that there is greater etched fissure conductivity than with traditional mineral acid application. It is noted that some care must be taken with the use of organic acids utilized downhole in the balance between etch properties and precipitation. It is understood that in carbonate formations that the increase in released calcium ions during carbonate rock dissolution can cause secondary precipitation, which can then end up clogging up equipment or formation pores that were being attempted to clear. Note that the combination of strong mineral acids with organic acids is used in wells with special types of tubing.
There is a need for the downhole application of a simpler acid application system that applies the power of strong mineral acids in a way that achieves the precision etching of organic acids. Such a system should be easy to use and improve etched fissure conductivity over conventional fracture acid treatments.